1. Field of the Invention
The present invention relates to a rolling motion stability control apparatus for a vehicle, particularly relates to an apparatus for restraining a roll increasing tendency of the vehicle to stabilize the rolling motion.
2. Description of the Related Arts
In order to maintain a stability of a vehicle, there is known heretofore an apparatus for controlling a braking force applied to each wheel by means of a braking system, as disclosed in the U.S. Pat. No. 6,086,168, for example. In the United States Patent, described is “For at least one vehicle-movement dynamics characteristic variable which is indicative of the tendency of the vehicle to tilt about the longitudinal axis, an associated tilting-prevention threshold value is prescribed. The respective characteristic-variable instantaneous value is acquired continuously and compared with the tilting-prevention threshold value. As soon as a characteristic-variable instantaneous value exceeds the associated tilting-prevention threshold value, the wheels which are on the outside during cornering are braked in order to prevent the vehicle tilting about the longitudinal axis of the vehicle.” Also, it is described that the instantaneous values of vehicle-movement dynamics characteristic variables include variables such as the lateral acceleration, the change over time of the lateral acceleration, the attitude angle, the attitude-angle velocity, the change in the attitude-angle velocity over time and the slip angle. And, as a first embodiment of the method, it is proposed that the lateral acceleration is the only vehicle-movement dynamics characteristic variable. With respect to another embodiment, it is described that the lateral acceleration, the change in the lateral acceleration over time, the attitude angle, the attitude-angle velocity, the change in the attitude-angle velocity over time and the slip angle are used as vehicle-movement dynamics characteristic variables. And, it is described that if one or more characteristic variables exceed the associated threshold values, the vehicle-movement dynamics control system brakes the wheels which are on the outside during cornering, such that they are placed in a high slip state.
According to the method as described in the above United States Patent, it focused on a vehicle behavior during cornering, which is a motion about a longitudinal axis of the vehicle, to be classified as a rolling motion in view of a vehicle motion control, as described in a book entitled “Vehicle Dynamics and Control” written by Masato Abe, and published by Sankaido Co. Ltd., on May 31, 1994. On Page 3 of that book, the rolling motion has been classified as a motion which is basically created by a steering operation, together with a lateral motion of the vehicle, and a yawing motion about a vertical axis of the vehicle. And, it is described on Page 148 of the book that if a rolling moment acts on the vehicle, a roll angle will be caused about a roll center.
In general, the roll angle has been employed as a factor indicative of the rolling motion of the vehicle, as described above. With respect to the roll angle, in Japanese Patent Laid-open Publication No. 2001-71787 and its English abstract of esp@cenet database, disclosed is a method proposed to improve the accuracy of deciding whether a vehicle is liable to roll over or not from its roll angle and roll angular velocity. It is described in the English abstract that with threshold lines plotted on a two-dimensional map employing parameters, or a vehicle roll angle and roll angular velocity, the hysteresis of the actual roll angle and roll angular velocity of the vehicle, when crossing either the threshold line from a non roll over region including the origin to a roll over region delimited away from the origin, shows that the vehicle is liable to roll over. According to the method as proposed in the above Japanese Publication, it is provided with a roll angular velocity sensor, which outputs the roll angular velocity, the integrated value of which is employed as a variation of the roll angle, and added to an initial value of the roll angle calculated on the basis of the output of a lateral acceleration sensor, thereby to obtain the roll angle. Then, it is described that the result determined whether there is a possibility of roll over of the vehicle can be used for controls for inflating an inflatable curtain, inflating a side air bag, extending a retractable roll over, or the like.
The method as proposed in the above Japanese Publication is applied to apparatuses which are adapted to perform the control for inflating the inflatable curtain or the like, in response to the result of determination based on the roll angle and roll angular velocity. Therefore, it is required to be capable of determining that the rolling motion of the vehicle has been excessive, to such an extent that vehicle passengers must be protected by inflating the inflatable curtain. On the contrary, it is not appropriate to inflate the inflatable curtain in such a state that the rolling motion of the vehicle is not so large. In contrast, when the rolling motion is determined for the stability control of the vehicle in the rolling motion, it is required to detect a slight rolling motion. Therefore, the method as described in the above Japanese Publication can not be applied to the determination of the rolling motion for the stability control of the vehicle according to the present invention.
In the U.S. Patent as described before, it is described that a plurality of vehicle-movement dynamics characteristic variables such as the lateral acceleration, and explained is one embodiment that employs the lateral acceleration as the only dynamics characteristic variable, and the other one embodiment that employs one or more characteristic variables as the dynamics characteristic variables. With respect to the former embodiment, however, described is “as soon as the characteristic-variable instantaneous value exceeds the associated tilting-prevention threshold value”, and with respect to the latter embodiment, described is “if one or more characteristic variables exceed the associated threshold values”. Thus, it is a fundamental requisite in either embodiment that the characteristic variables are compared with the associated threshold values, respectively. Therefore, even if it was possible to apply the one or more characteristic variables to the method for determining the tendency of the vehicle to tilt about the longitudinal axis, as proposed in the above-described U.S. Patent, it would not be possible to appropriately determine the rolling motion to be used in the vehicle motion stability control according to the present invention. Also, it is important to determine how a braking force control is to be continued, or to be terminated.